If you and a friend are in the same room, the Xbox Kinect sensor can only track and provide feedback for one person at a time.
However, with Xbox LIVE, you and a friend in a different location can work out together with a live video feed through Nike+ Kinect Training's "Workout With a Friend" feature.
Access the feature from the Main Menu of Nike+ Kinect Training by swiping "Group Training" and selecting "Workout With a Friend".
To use "Workout With A Friend", you and your friend must be Xbox LIVE friends, must have copies of Nike+ Kinect Training, and be connected to Xbox LIVE.
Once connected, the host will choose the type, difficulty, and length of workout you and your friend do together. The "Workout With a Friend" feature also uses Kinect Chat technology.